Circular waveguide dual-mode filters are very widely used in multiplexers on-board telecommunications satellites. Manufacture thereof requires a complicated step consisting in manually tuning the filter, thus giving rise to high cost and lengthy development time.
In the basic configuration, two resonances are excited in the same cylindrical waveguide cavity, thereby enabling the dimensions of the device to be reduced. Another advantage of that known configuration is that it is possible to use coupling between non-adjacent cavities and also to achieve transmission zeros. By way of example, this is described in an article by A. E. Williams entitled "A four-cavity elliptic waveguide filter", published in IEEE Transactions MTT-18, December 1970, pages 1100 to 1104, and in an article by A. E. Atia et al., entitled "Narrow-bandpass waveguide filters", published in IEEE Transactions MTT-20, April 1972, pages 258 to 265.
Cross coupling can be obtained within each cavity by means of an adjustment screw, and cross coupling can also be obtained between adjacent cavities by means of a cross-shaped iris. In addition, to enable each resonance to be synchronous, i.e. for all of the cavities to have the same resonant frequency, other adjustment screws are added for independent adjustment, and in particular for frequency tuning. As a result, a filter generally has at least three adjustment screws per cavity, and each of the screws needs to be adjusted manually.
To reduce or eliminate this adjustment step, it is necessary to have available a complete wave representation of the adjustment screws. Although that is theoretically possible, e.g. by implementing the method of finite elements, the computation time required by such a concept is unacceptable in practice.
Proposals have recently been made in an article by M. Guglielmi et al., entitled "Dual-mode filters without tuning screws", published in IEEE Microwave and Guided Wave Letters, Vol. 2, No. 11, November 1992, pages 457 and 458, for solving the problem by using a waveguide having circular ribs. The article by Xiao-Peng Liang entitled "Dual-mode coupling by square corner cut in resonator and filters", published in IEEE Transactions on Microwave Theory and Techniques, Vol. 40, No. 12, December 1992, pages 2294 to 2302, proposes a waveguide having a square-shaped cutout. Both of those two solutions are of interest, but they suffer from the drawback of requiring non-standard waveguide mode analysis which makes use of computation that is very complicated and requires very long computation time.
More recently, other shapes have been proposed for the purpose of eliminating the need for adjustment elements, in an article by R. Orta et al., entitled "A new configuration of dual-mode rectangular waveguide filters", published at pages 538 to 542 of the Proceedings of the 1995 European Microwave Conference held at Bologne, Italy, and in an article by S. Moretti et al., entitled "Field theory design of a novel circular waveguide dual-mode filter", published at pages 779 to 783 of the Proceedings of the 1995 European Microwave Conference at Bologne in Italy. Nevertheless, it is necessary in practice to have control over the resonant frequency so as to compensate for manufacturing tolerances and so as to enable very accurate frequency adjustment to be performed, as is required in narrow band applications.